As shown in FIG. 1, a plastic chair made by injection molding of the prior art comprises a back 12, a seat 11 and a plurality of legs. Though it is easy and cheap to manufacture, the chair has the following disadvantages. Firstly, since the flexibility of the back is poor, it is not comfortable to sit on for a long period of time. Secondly, the chair is not ventilative enough, since too many ventilating holes are not allowed due to structure-strength consideration.